1. Field of Invention
The present invention relates to an electromagnetic band-gap structure. More particularly, the present invention relates to a planar antenna with an electromagnetic band-gap structure.
2. Description of Related Art
Due to rapid development in information technology, applications on electronic devices have become more complex and are able to do more things. Therefore electronic devices such as notebooks, personal digital assistants (PDA) etc. have been frequently used in our daily life. This not only provides increased convenience and efficiency, but also causes the compression of time and space, where the culture and information exchange is becoming more frequent than ever before so as to achieve the optimal welfare for all mankind. Accordingly, the antenna plays an important role in communication and transportation technology applications, spreading the messages and knowledge more conveniently.
In antenna design, the metal plate is often used as the reflector plane or ground plane in the antenna so as to constitute a perfect electric conductor. However, if the metal plate and the antenna are getting too close, there would exist an image current on the metal plate, flowing in a direction opposite that of the current on the antenna, and therefore the current might cancel with each other so as to result in a poor radiation efficiency and antenna gain. Consequently, the distance between the antenna and metal ground plane should be large enough to increase antenna gain and lower the backward radiation in order to reduce unnecessary loss.
As the evolution of the communication product is to be smaller and shorter, the in exact height of the antenna is determined by the height of the product, resulting in a demand for the low profile design and the minimum size. In addition, the embedded antenna applied in the notebook or PDA is principally disposed on the fringe of the screen, which is narrow area with a limited width, so that the antenna and the ground plane are too close to maintain the radiation efficiency and communication quality. In the former method, the two dimension electromagnetic band-gap structure is adopted to be equivalent to a parallel LC resonant circuit at the corresponding operating frequency. When the parallel LC resonant circuit resonates, the impedance would become extremely high so as to make the electromagnetic band-gap structure work. However, with the use of the structure having a high impedance surface, the image current would have the same phase with the antenna in order to maintain the original characteristic of antenna and achieve the demand of the low profile design. In the general application, a large area of the two dimension electromagnetic band-gap structure is basically needed. Furthermore while designed with the antenna, the electromagnetic band-gap structure would become a three-dimension structure. As a result, the two dimension electromagnetic band-gap structure is insufficient for practical use.
Accordingly, what we need the most is a one dimension electromagnetic band-gap structure, used for being equivalent to a perfect magnetic conductor at the corresponding operating frequency. In accordance with the features, it cannot only shorten the distance between the antenna and the ground plane and maintain the characteristics of the antenna, but also be embedded in the small-size wireless communication products.